1. Field of the Invention
This invention relates to an apparatus and method for displaying an input signal waveform and, in particular, relates to an apparatus for displaying the pedestal and top portions of a waveform of an input signal on a display screen even if the amplitude of the input signal is multiplied to obtain improved resolution in the display screen.
2. Description of the Related Art
FIG. 1(A) illustrates a display screen DS wherein a signal waveform on a horizontal line of a video signal is schematically shown. When it is required to measure a voltage difference between pedestal and white levels of the video signal, a reference cursor K.sub.REF is adjusted to be located at the pedestal level and a delta cursor K.sub.DELT is adjusted to be located at the white level, thereby the difference level between the cursors, and hence between the pedestal and white levels of the video signal may be numerically displayed on the screen. In this case, since a magnification ratio of the video signal is low, for example "1", it is sometimes probable that the reference cursor K.sub.REF and the delta cursor K.sub.DELT will only be locatable in the neighborhood of the pedestal and white levels of the video signal. Thus, in such a case, a magnitude of the video signal is multiplied or amplified by a factor larger than "1" and then the amplified waveform is displayed on the screen DS to improve the resolution so that the cursors K.sub.REF and K.sub.DELT are precisely located at the pedestal and white levels of the video signal.
When the magnitude of the video signal is multiplied by a certain factor and displayed, the higher level portion of the signal waveform is usually off the screen DS together with the delta cursor K.sub.DELT, as shown in FIG. 1(B). Therefore, after the precise location of the reference cursor K.sub.REF at the pedestal level is executed as shown in FIG. 1(B), the screen must be scrolled by means of a vertical location arranger to reenter the delta cursor K.sub.DELT in the screen DS together with the higher level portion of the waveform, as shown in FIG. 1(C), thereby the delta cursor K.sub.DELT is able to be accurately located at the white level. Further, since only one of the two cursors can be displayed on the same screen, it is impossible to observe on the screen DS when the cursor located off the screen is moved from the pedestal or white level of the video signal from a precise location. Thus when it is necessary to know the precise location of the off-screen cursor, scrolling must be carried out which makes the operation complicated. This problems may be caused on signals other than the video signal.